1. Field of the invention
The invention relates to an installation for spraying coating products such as paint of the type in which the object to be coated, carried by a conveyor, passes in front of the working area of a manipulator robot carrying a sprayer which sprays the coating product in the form of fine particles.
The invention is more particularly directed to a new arrangement making it possible to solve the problem of changing color when the objects passing in succession in front of the robot have to be painted different colors and the problem of providing the necessary galvanic isolation between the means for distributing the coating products and the sprayer means where the paint used has a low resitivity (water-based paint) and when the spraying means incorporate a high-tension voltage source for implementing electrostatic spraying.
2. Description of the prior art
A paint spraying installation in a production unit as large as an automobile factory, for example, generally comprises several closed loop paint distribution circuits which are very long (these circuits may pass right across part of the plant) and which make the connections between large paint storage tanks and the various spraying booths. It is therefore necessary to provide a circuit of this kind for each color and another circuit of the same kind for the solvent or cleaning product. For obvious safety reasons these circuits are electrically grounded.
In a spray booth supplied in this way, the objects to be painted (which are automobile bodies in the example under discussion) are carried by conveyor means through the booth in which is at least one robot carrying a paint sprayer and able to operate within a particular "working area" inside the booth. Use is routinely made of robots having up to six or even seven degrees of freedom or axes so that the sprayer can be oriented at will and caused to penetrate into certain less accessible parts of the object to be painted. For simple shapes a robot with only three of four degrees of freedom may be adequate.
The sprayer may be of the electrostatic type involving high rotation speeds, or of the pneumatic or hydrostatic type. The robot usually comprises an arm made up of several segments articulated to each other, the arm possibly being carried by a chassis movable lengthwise of the conveyor means. One of the usual problems to be overcome in this type of installation is that of changing color between two consecutive objects. In the automobile industry in particular there is no question of painting long series of bodies the same color. To the contrary, the more usual situation is that where the color has to be changed after virtually every body. This implies the possibility of implementing extremely rapid rinsing and drying cycles for the spraying means. To give an example, it may be necessary to change the color about once a minute and all the operations necessary for changing color can take about ten seconds.
In conventional installations as currently known all the coating product distribution circuits, a compressed air supply circuit and a cleaning product circuit are connected through selectively operable isolating valves to a manifold which has a common outlet branch connected to the sprayer.
To change color it is necessary to shut off the valve in the circuit for the coating product currently being used a particular (computed) time before the end of the current spraying phase and then to open the compressed air valve to expel the remaining product through the sprayer. The robot is then moved away from the object and oriented towards a recovery receptacle. The cleaning product valve is opened until the manifold and the sprayer are clean. This valve is then closed and the compressed air valve is opened again to expell the cleaning product contained in the manifold into the recovery receptacle. The opening of the compressed air valve is prolonged to dry the conduits and then, once it has been closed, the valve on another coating product circuit is opened to fill the manifold and the conduits until a little of the new color coating product is expelled from the sprayer. The robot is then turned back to face the new object to be sprayed.
If the installation comprises only a small number of different color coating products, three or four, for example, the manifold may be disposed relatively near the sprayer, on one of the final articulated segments of the robot arm, for example, each coating product being supplied through a flexible hose. On the other hand, if there are too many different coating products (there may be up to 20) this solution is no longer practicable and the manifold must then be fixed with its outlet connected to the sprayer by a single flexible hose. If the objects to be sprayed are bulky, which is the case with automobile bodies, the robots have to move over great distances parallel to the largest dimension of the objects to be sprayed, which may be five or six meters, for example. In this case the manifold is far away from the sprayer, up to ten meters, for example, which considerably increases the volumes of conduit to be cleaned and therefore the length of the color change sequences. It often becomes necessary in such cases to duplicate the supply circuits (manifolds, valves, conduits) in order to use part for spraying while the other part is undergoing a cleaning and color change cycle. Also, the quantities of the coating product and of the cleaning product wasted on each color change are important, in the order of several hundred cubic centimeters.
Furthermore, the installation is made more complex by the fact that, in order to procure a constant flowrate of the coating product during the end of the spraying phase in which the coating product is expelled by compressed air, it is necessary to provide a pressure regulator immediately upstream of the sprayer on the coating product circuit. Finally, as the sprayers generally have very small cross-section outlet orifices, to secure correct spraying, it is not always possible to eject all of the cleaning product through these orifices. For the cleaning of the manifold and the conduits to be effective and fast it is necessary to maintain turbulent flow and therefore a high speed combined with a high flowrate. To this end it is necessary to provide between the pressure regulator and the sprayer a large cross-section purging circuit and a purging valve through which the cleaning product flows to the recovery receptacle. Also, the pressure regulator and the purge valve being in the immediate vicinity of the sprayer, in an area that may contain a flammable gaseous mixture and that may be at a high-tension voltage when the sprayer is of the electrostatic type, this device is usually actuated by compressed air, calling for additional pneumatic hoses.
In the case of an electrostatic installation all these problems concerning color changes are accentuated where there is a requirement to use coating products of low resistivity, such as water-based paint, for example, while conserving the advantages of electrostatic spraying. In this case the sprayer is often held at a high-tension voltage and it is necessary to avoid short-circuits between the sprayer and the coating product distribution circuits that are electrically grounded. For the leakage current to be acceptable with the water-based paints currently used it would be necessary to use insulative conduits with a length/cross-section ratio that would be unacceptable because it would lead to extremely long color changing times and unrealistically large quantities of coating and cleaning product being wasted.
Also, when the coating product supply conduits and the purge circuits are filled with electrically conductive product,. the dielectric strength of the conduits must be sufficiently high to withstand the high-tension voltage applied. Also, the quantity of product that they contain represents a relatively large electrical capacity able to store a quantity of energy much higher than the tolerated limits.
To confront the problem specific to changing color with low-resistivity coating products, French Patent No. 2 572 662 proposes to fill an intermediate storage tank with just the quantity of paint needed for each application. This intermediate storage tank is fixed so that each color change entails cleaning out not only the storage tank but also all the conduits connected to it. Furthermore, in one embodiment isolation is re-established after filling by draining and drying a sufficient length of conduit upstream of the intermediate storage tank. This operation requires a prohibitive length of time on each color change.
The invention proposes to solve the problems of changing color by considerably simplifying the equipment situated between the distribution circuits and the sprayer, irrespective of the type of coating product used.
The invention also proposes to reduce the time necessary to change color, even when the number of different colors is large and the robot which moves the sprayer has to move over long distances because of the size of the objects to be sprayed.
In the specific instance of electrostatic spraying of low-resistivity coating products, the invention further proposes to solve in a particularly simple way the problem of isolating the electrostatic sprayer from the coating product distribution circuits, that are electrically grounded.